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Trends in Horticulture (2023) Volume 6 Issue 2
doi: 10.24294/th.v6i2.3185
1
Review Article
Effect of different pre-treatments on rehydration kinetics of solar and
hot-air dried Fuji apple slices
Debashish Dey, Kshanaprava Dhalsamant, Punyadarshini Punam Tripathy*
Agricultural and Food Engineering Department, Indian Institute of Technology Kharagpur, West Bengal, 721302, India
* Corresponding author: Punyadarshini Punam Tripathy, punam@agfe.iitkgp.ac.in
ABSTRACT
The present study demonstrates the effect of direct solar drying (DSD) and hot air drying (HAD) on the quality
attributes of Fuji apple slices. DSD samples took a longer time (150–180 min) to dry and simultaneously reached higher
equilibrium moisture content at the end of rehydration than HAD samples. DSD samples have higher rehydration ability,
dry matter holding capacity, and water absorption capacity than HAD samples. Among several empirical models, the
Weibull model is the best fit with higher R2 (0.9977), lower root mean square (0.0029), and chi-square error (0.0031) for
describing the rehydration kinetics. Rehydrated HAD samples showed better color characteristics than DSD in terms of
overall color change, chroma, and hue angle values. Whereas the hardness and chewiness of rehydrated DSD samples
were better than HAD samples because of higher dry matter holding capacity in DSD. Apart from color retention, the
DSD samples showed better rehydration capacity and a good texture upon rehydration than HAD slices.
Keywords: solar drying; apple slices; rehydration indices; thin layer modeling; color; hardness
1. Introduction
Apple is a good source of dietary fiber, energy, calcium, iron,
potassium, vitamin A, vitamin C, and also some amount of proteins[1,2]
which helps to maintain good health. Drying is an important process that
involves moisture removal from a food product to decrease microbial
attack and increase shelf life. Drying induces various structural changes
in the food product based on the drying conditions, which further
influences the rehydration ability of the food[3–5]. Generally, drying is
carried out in conventional dryers that are costly and use fossil fuels[6].
Therefore, more highlighting is given on solar drying nowadays.
However, the solar drying technique takes a little longer time in
comparison to other drying techniques to reach the desired moisture
content[7]. The different types of solar dryers are, direct (commodity
receives direct sunlight), indirect (commodity receives heat from
collector being heated from sunlight), and mixed (mix of direct and
indirect), and hybrid type (advanced version)[8].
Solar drying and hot air drying are two methods used for food and
material preservation, each with its own set of advantages and
disadvantages[9]. Solar drying harnesses the sun’s energy, making it eco-
friendly and cost-effective. It reduces dependency on conventional
energy sources, promotes sustainability, and is ideal for small-scale
operations[6]. However, its efficiency is weather-dependent, impacting
drying rates. On the other hand, hot air drying offers consistent results
regardless of weather conditions. It’s faster, and applicable to various
ARTICLE INFO
Received: 6 November 2023
Accepted: 15 November 2023
Available online: 17 November 2023
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2
scales, but may consume more energy and incur higher costs. Balancing these methods depends on factors like
scale, location, and resource availability[10].
Rehydration involves three concomitant processes namely, water absorption into the dried cellular matrix,
swelling of the dried matrix, and the leaching of soluble solutes[11]. It involves two cross-current mass fluxes;
one is water flux which enters the dried matrix from the rehydration solution and the other is the flux of soluble
solutes which comes out from the rehydrating product because of leaching. Various factors influence the
process of rehydration such as pre-drying treatments, drying conditions and techniques, immersion media’s
composition, rehydration temperature, etc. All these factors induce irreversible changes in the product and
because of these changes, the equilibrium moisture content attained after rehydration is not the same as that of
the original product[12].
Apples are a rich source of polyphenols and hence it is highly susceptible to browning, therefore are need
to be pre-treated before drying[13]. Pre-treatment can be done either by a physical method such as blanching or
chemical methods which includes the application of chemicals[14,15]. Similar results were reported by
Doymaz[16] for the blanching of Amasya red apples, and Lewicki et al.[3] by treating apple slices with 0.5%
ascorbic acid solution to limit enzymatic browning.
However, any study regarding rehydration of Fuji apple slices has not been thoroughly researched in the
literature. Therefore, in the present study rehydration characteristics of Fuji apple slices were studied with
different pretreatments and drying conditions viz. solar and hot air drying. Additionally, the rehydration
kinetics and the effect of drying techniques on the color and texture of Fuji apple slices were also investigated.
2. Materials and methods
Fuji apples (Malus domestica) were procured from the community market of IIT Kharagpur, India. Apple
slices of (30 mm × 5 mm) were prepared from raw apples with an average weight of 3.05 ± 0.35 g each. Pre-
treatments such as blanching at 70 °C and dipping in ascorbic acid (0.5%) were applied before drying for 2
min each to eliminate the effect of enzymatic activity and browning. Some samples were kept without
pretreatment termed as un-treated and used for comparison purposes.
2.1. Experimentation
Drying experiments were carried out in a direct solar dryer (DSD) (in-house built) and laboratory-scale
hot air dryer (HAD) (Plausible Instruments Private Limited, Delhi). The natural convection direct solar dryer
consists of an inner black absorbing surface that collects the light and converts it to heat. It had opposite inlet
and outlet vents for the flow of air and also a top glass cover that traps the light inside the drier further
increasing the temperature inside it. The products to be dried were kept on a wire mesh tray inside the dryer.
The experiments were performed on a bright sunny day with an average solar intensity level of 600 W/m2,
relative humidity of 40 ± 0.5% inside the dryer and the air velocity inside solar dryer was negligible. The hot
air dryer consisted of an air temperature electronic controller, an air blower, an electric heater, and a balance
heater for increasing the temperature rapidly initially. The dryer was sustained at a relative humidity of 22.45
± 0.64%, air velocity of 1.3 ± 0.16 m/s, and 60 °C temperature.
The dried samples were then rehydrated in a constant temperature water (EIE Instruments PVT LTD)
bath having three heaters for attaining the desired temperature and an electronic proportional controller for
controlling the temperature.
2.2. Instrumentation and measurements
Pyranometer (Delta OHM, Model: LP PYRA-03; Italy) was used for measuring solar radiation intensity,
and the weight of samples was measured by an electronic weighing balance (Sartorius BSA 2202S, accuracy
± 0.01 g; Goettingen, Germany) was used for measuring weight. The air-flow rate and humidity inside the
dryer were measured using a probe anemometer (AM-4213, Lutron, Taiwan) and a probe humidity meter (HT-
3
305, Lutron, Taiwan), respectively. A colorimeter (KONICA MINOLTA, Osaka, Japan) was used for
measuring color and the texture was measured by a texture analyzer (Brookfield texture analyzer CT3 version
2.1, US).
2.3. Drying kinetics of apple slices
The moisture content of fresh apples was determined as 86.88 ± 1.59% (wb) by the oven drying method
(AOAC 925.09, 2002). All the samples were dried until a constant weight of the samples was achieved (which
was around a moisture content of less than 3% (wb))[17,18]. The samples were then cooled and kept in sealable
low-density polythene pouches and were used for rehydration experiments at 30 and 50 °C temperatures
afterward. The moisture content of dried samples has been converted to moisture ratio (MR) as follows[19]:
MR
=
M
−
M
M
–
M
(1)
where Me, Mt, and M0 are equilibrium moisture content (db), moisture content at any time during drying (db),
and initial moisture content (db) of the sample, respectively.
2.4. Rehydration of dried apple slices
The HAD and DSD dried apple slices were rehydrated using a constant temperature water bath with solid
to liquid ratio of 1:50. The dried samples were placed in petri dishes containing distilled water and kept inside
the constant temperature water bath at temperatures of 30 °C and 50 °C. The sample weight was measured at
every 15-min time interval until it reached the equilibrium moisture content. The blotting of samples was done
with tissue paper to remove the surface water before measuring weight. The ability of the samples to imbibe
water was quantified in terms of rehydration ratio and was calculated as follows[20]:
Rehydration ratio
=
Weight of rehydrated sample
(
g
)
Weight of dried sample
(
g
)
(2)
2.5. Modeling of rehydration kinetics
To describe the kinetics of the rehydration of Fuji apple slices, three empirical models were employed as
given in Table 1. The moisture sorption curves for rehydration have been described by a non-exponential
equation of the Peleg model for several types of commodities[21–24]. The constants k1 (min kg d.m./kg water)
and k2 (kg d.m./kg water) are two parameters of Peleg’s model. Weibull model as explained by Dr. Walodi
Weibull represents the breaking strength distribution of the materials and it explains the working pattern of
systems that have a degree of variability[25]. The constants α (dimensionless) and β (min) are two parameters
of the Weibull model. For describing the mass transfer phenomenon of the process of rehydration, a first-order
kinetic model (FOKM) was also considered to carry out the modeling[26], in which kr represents the rate of
rehydration (/min).
Table 1. Mathematical models applied to rehydration of Fuji apple slices.
Model name Model
Peleg model
M
=
M
+
t
k
+
k
t
First order kinetic model (FOKM)
M
–
M
M
–
M
=
e
–
k
Weibull model
M
–
M
M
–
M
=
1
–
e
β
α
2.6. Assessment of rehydration indices
During rehydration, leaching of soluble solids from the cellular matrix of the sample leads to significant
loss of minerals, amino acids, sugars, and vitamins[3,27]. Three indices were used for the estimation of the
4
rehydration characteristics of dried foods[3], viz., rehydration ability (RA), dry matter holding capacity (DHC),
and water absorption capacity (WAC). WAC as given in Equation (3) varies in the range from 0 to 1 and is
the measure of the capability of the dried matrix to imbibe water for the water loss during the drying process[28].
WAC
=
M
(
1
−
S
)
–
M
(
1
−
S
)
M
(
1
−
S
)
–
M
(
1
−
S
)
(3)
where Mr, Md, and M0 represent the mass of the rehydrated sample, dried sample, and fresh sample (before
drying), respectively, and Sr, Sd, and S0 represent the percentage dry matter content of the rehydrated sample,
dried sample, and fresh sample, respectively.
DHC measures the capacity of the material to hold its soluble solids after rehydration and shows a clear
indication of the amount of damage in the tissue of the materials during drying. DHC varies in the range of 0
to 1 and can be computed as follows[9]:
DHC
=
M
S
M
S
(4)
Rehydration ability RA as given in Equation (5) is the measure of the capacity of the dried material to
rehydrate by accounting for the capacity to absorb water and losses due to leaching of soluble solids and varies
in the range of 0 to 1[9].
2.7. Color and texture measurements
RA
=
WAC . DHC
(5)
Color is an important attribute essential for determining the quality of a product. Color measurements of
rehydrated apple slices were done (Konica Minolta Colorimeter, Europe) in terms of overall color change (∆E),
chroma (C*), and hue angle (h*) in view of the basic color combination of lightness (L*), redness (a*), and
yellowness indexes (b*) as given in Equations (6)–(8)[29]:
Δ
E
=
(
Δ
L
∗
)
+
(
Δ
a
∗
)
+
(
Δ
b
∗
)
(6)
C
∗
=
(
a
∗
)
+
(
b
∗
)
(7)
h
∗
=
ta
n
–
1
b
∗
a
∗
(8)
where ∆L*, ∆a* and ∆b* indicate the lightness, redness, and yellowness index changes of rehydrated slices with
respect to fresh slices.
The textural characteristics of the rehydrated samples were measured in terms of chewiness and hardness
using the texture analyzer (TA-XTplus, Stable Micro Systems Products). The measurement was performed at
a constant speed of 1 mm s–1 using a 6 mm diameter cylindrical puncture probe.
2.8. Statistical analysis
The experiments were carried out in triplicates. The accuracy of different rehydration models was
analyzed statistically using the root mean square error (RMSE), chi-square (χ2), and coefficient of
determination (R2), as given in Equations (9)–(12)[30]. The experimental drying and rehydration data were
analyzed using ANOVA. The significance of difference was determined by a one-factor analysis of variance
(p ≤ 0.05).
RMSE
=
1
𝑛
(
𝑀
exp
−
M
pre
)
0.5
(9)
5
χ
=
(
M
exp
–
M
pre
)
M
pre
(10)
R
=
1
–
∑
M
exp
–
M
pre
∑
M
exp
–
M
mean
(11)
M
mean
=
∑
M
exp
i
1
n
(12)
where n is the number of experimental data points, and Mpre and Mexp are predicted and experimental values,
respectively. The best-fit model was considered based on the least χ2 and RMSE, and highest R2 values.
3. Results and discussion
3.1. Drying kinetics of apple slices
Figure 1 shows the drying kinetics of un-treated and pre-treated apple slices dried in DSD and HAD,
respectively. Figure 2 represents the variation in the rate of drying as a function of the moisture ratio of DSD
and HAD samples. It could be noticed that the moisture content of the samples decreased with time and the
constant rate drying period was not detected. The falling period range involves the entire drying process. A
similar type of result was concluded by Doymaz[16] in apple slices during the hot-air drying. It can also be
Figure 1. Drying kinetics of un-treated and pre-treated Fuji apple slices dried in (a) direct solar dryer and (b) hot air dryer.
Figure 2. Variation in drying rate as a function of moisture ratio (a) direct solar dryer and (b) hot air dryer.
6
noticed that the various pre-treatments had a notable effect on the drying time of apple slices as the blanched
samples dried faster (105 min for HAD and 150 min for DSD) than the ones which are un-treated (150 min for
HAD and 180 min for DSD) and pre-treated with ascorbic acid (135 min for HAD and 165 min for DSD) in
both HAD and DSD samples. The blanched, ascorbic acid pre-treated, and un-treated samples took 105, 135,
and 150 min, respectively in HAD and a slightly longer time of 150, 165, and 180 min, respectively in DSD
to obtain the anticipated moisture content. The reason behind this is that solar drying works on the principle of
natural convection and so it takes a little longer time to dry a particular product as compared to hot air drying
which is based upon the principle of forced convection (and hence takes less time).
3.2. Rehydration kinetics
3.2.1. Rehydration kinetics of DSD and HAD apple slices
The rehydration kinetics of blanched, ascorbic acid pre-treated, and un-treated solar dried and hot air-
dried apple slices at 30 and 50 °C, respectively are shown in Figure 3. In terms of final EMC attained at the
end of the rehydration process, blanched samples attained the highest moisture content (3.61 and 3.79 kg/kg
d.b. at 30 and 50 °C, respectively for HAD samples and 3.73 and 4.09 kg/kg d.b. at 30 and 50 °C for DSD
samples), followed by ascorbic acid pre-treated (3.10 and 3.41 kg/kg d.b. at 30 and 50 °C, respectively for
HAD samples and 3.33 and 3.62 kg/kg d.b. at 30 and 50 °C for DSD samples) and un-treated samples (2.79
and 2.98 kg/kg d.b. at 30 and 50 °C, respectively for HAD samples and 3.01 and 3.21 kg/kg d.b. at 30 and
50 °C for DSD samples). This trend was obtained for samples rehydrated at both 30 and 50 °C rehydration
temperatures. In terms of time taken for the samples to attain EMC, blanched samples took a long time (180
min and 165 min at 30 and 50 °C for HAD samples and 195 min and 180 min at 30 and 50 °C for DSD samples,
respectively) as compared to ascorbic acid pre-treated (165 min and 135 min at 30 and 50 °C for HAD samples
Figure 3. Rehydration kinetics of solar dried (a) and (b) and hot air-dried (c) and (d) apple slices rehydrated at 30 °C and 50 °C,
respectively.
7
and 180 min and 150 min at 30 and 50 °C for DSD samples, respectively) and un-treated samples (150 min
and 120 min at 30 and 50 °C for HAD samples and 165 min and 135 min at 30 and 50 °C for DSD samples,
respectively) for both the rehydration temperatures. It can also be observed that each of the samples rehydrated
at 50 °C attained higher moisture content (3.79, 3.41 and 2.98 kg/kg d.b. for blanched, ascorbic acid treated,
un-treated HAD samples and 4.09, 3.62 and 3.21 kg/kg d.b. for blanched, ascorbic acid treated, un-treated
DSD samples, respectively) than those at 30 °C (3.61, 3.10 and 2.79 kg/kg d.b. for blanched, ascorbic acid
treated, un-treated HAD samples and 3.73, 3.33 and 3.01 kg/kg d.b. for blanched, ascorbic acid treated, un-
treated DSD samples, respectively). The reason behind this could be the more swelling of the cellular matrix
in a shorter time interval with an increase in rehydration temperature. Thus, it could be concluded that the
water absorption rate of dried slices increased with rehydration at higher temperatures because of the rate of
increment in diffusion of water[9]. The blanched slices showed 10.72% and 19.30% higher water absorption
capacity than ascorbic acid pre-treated and un-treated samples, respectively at 30 °C and 11.49% and 21.51%,
respectively at 50 °C.
A similar tendency was obtained as in the case of DSD apple slices and the blanched samples were
reported to attain higher final EMC than the ascorbic acid-treated and un-treated samples for both the
rehydration temperatures. Each of the rehydrated samples at 50 °C gained higher final equilibrium moisture
content in a shorter time as compared to the samples rehydrated at 30 °C. The blanched sample was observed
to be superior in terms of higher water absorption capacity, i.e., 14.12% and 22.71% more at 30 °C, and,
10.02% and 21.37% more at 50 °C than the ascorbic acid pre-treated and un-treated samples, respectively.
3.2.2. Rehydration ratio of DSD and HAD apple slices
Figure 4 represents the variation of the rehydration ratio of solar-dried and hot-air dried slices with time
at 30 and 50 °C, respectively. It can be depicted that blanched samples (4.12 at 30 °C and 4.33 at 50 °C for
Figure 4. Variation of rehydration ratio of solar dried (a) and (b) and hot air-dried (c) and (d) apple slices rehydrated at 30 °C and
50 °C, respectively.
8
HAD samples and 4.23 at 30 °C and 4.51 at 50 °C for DSD samples) attained a higher rehydration ratio than
ascorbic acid pre-treated (3.51 at 30 °C and 3.92 at 50 °C for HAD samples and 3.83 at 30 °C and 3.98 at 50 °C
for DSD samples) and un-treated samples (3.27 at 30 °C and 3.64 at 50 °C for HAD samples and 3.69 at 30 °C
and 3.74 at 50 °C for DSD samples). This may be because of the loosening of the cellular matrix due to
blanching hydro-thermal treatment thereby increasing the overall size of pores which aided in reducing drying
time and increased water uptake while rehydration. Ascorbic acid treatment on the other hand can be
considered as a surface phenomenon (in comparison to blanching which is a bulk phenomenon), which when
applied to the samples creates partial osmosis because of which water comes out from the matrix thus widening
the pores due to osmotic pressure difference. Since the ascorbic acid treatment only resulted in a widening of
the surface pores, it follows that the ascorbic acid-treated samples had a lower equilibrium moisture content
than the blanched ones. Instead, the low number of enlarged pores in un-treated samples meant that they could
not absorb more water throughout the rehydration procedure.
3.2.3. Comparison between rehydration characteristics of DSD and HAD apple slices
The rehydration kinetics of DSD and HAD blanched samples for the two different rehydration
temperatures and the variation of rehydration ratio with time for the same have been shown in Figure 5. The
figure depicted that both HAD and DSD blanched samples, showed maximum water absorption capability for
the two rehydration temperatures (3.73 kg/kg d.b. at 30°C, 4.09 kg/kg d.b. at 50 °C for DSD blanched samples
and 3.61 kg/kg d.b. at 30 °C, 3.79 kg/kg d.b. at 50 °C for HAD blanched samples). The rehydration ratio is the
usual expression of the rehydration ability of dried apple slices[11], therefore the optimization of the rehydration
process was done based on the rehydration ratio of HAD and DSD samples. Samples that achieved higher
rehydration capacity among all other samples, i.e., blanched HAD and DSD samples for both the rehydration
temperatures have been selected for further analysis. From these figures, it could be anticipated that with an
increment in the rehydration temperature, there is a significant increase in the water absorption capability for
both DSD (3.73 to 4.09 kg/kg d.b.) and HAD (3.61 to 3.79 kg/kg d.b.) blanched samples. Figure 6 represents
the variation in rehydration kinetics of DSD and HAD blanched samples at 30 and 50 °C, respectively. It could
be deduced that for both the rehydration temperatures, DSD blanched slices gained higher EMC (3.73 and 4.09
at 30 and 50 °C, respectively) as compared to HAD blanched samples (3.61 and 3.79 at 30 and 50 °C
respectively). The reason behind this could be the more structural disruption in HAD samples making them
less efficient in attaining higher EMC as compared to DSD samples as the latter is based on the concept of
natural convection where the samples are subjected to the natural flow of hot air, whereas in a HAD, the air is
circulated by blower and forced convection prevails. Since DSD samples suffered less cellular damage, they
were able to soak up more water for a longer period of time, which may have contributed to the longer time
required to reach the EMC.
3.3. Rehydration indices
The values of water absorption capacity, dry matter holding capacity, and rehydration ability of DSD and
HAD blanched samples rehydrated at 30 and 50 °C have been given in Table 2. It is observed from the table
that the DSD apple slices have higher water absorption capacity (0.592 and 0.658, at 30 and 50 °C, respectively)
than HAD samples (0.577 and 0.632 at 30 and 50 °C, respectively) for both the rehydration temperatures.
Also, DSD samples have been found to possess more dry matter holding capacity (0.300 and 0.286 at 30 and
50 °C, respectively) because of minimum cellular damage while drying due to which samples tend to absorb
more water and also retain more soluble solids as compared to HAD samples (0.259 and 0.246 at 30 and 50 °C,
respectively). It can be seen from the table that with an increase in rehydration temperature, dry matter holding
capacity value decreases. This is because, with an increase in soaking temperature, the cellular matrix of apple
slices widens up much faster, which ultimately leads to more flux of soluble solids coming out from the
samples to the rehydrating solution causing a decrease in dry matter holding capacity. The higher dry matter
holding capacity values of DSD samples mean that the samples will be high in nutrient content as compared
9
Figure 5. Rehydration kinetics (a) and (b) and rehydration ratio (c) and (d) of blanched samples dried in direct solar dryer (a) and (c)
and hot air dryer (b) and (d) at 30 °C and 50 °C.
Figure 6. Variation in rehydration kinetics (a) and (b) and rehydration ratio (c) and (d) of direct solar dryer and hot air dryer
blanched samples rehydrated at 30 °C and 50 °C, respectively.
10
Table 2. Rehydration indices of hot air dried and solar dried blanched Fuji apple slices rehydrated at 30 °C and 50 °C.
Rehydration temperature (oC)
Drying technique
WAC DHC RA
30 SD 0.592 ± 0.027bc
0.300 ± 0.011a
0.178 ± 0.009ab
HAD 0.577 ± 0.006cd
0.259 ± 0.009b
0.150 ± 0.007cd
50 SD 0.658 ± 0.027a
0.286 ± 0.010a
0.189 ± 0.014a
HAD 0.632 ± 0.018ab
0.246 ± 0.019bc
0.155 ± 0.011c
a,b,c,d: Values in the same column with different letters represents significant difference (p ≤ 0.05).
to HAD samples. The overall rehydration ability values were found to be higher for DSD blanched samples
(0.178 and 0.189 at 30 and 50 °C, respectively) as compared to HAD blanched samples (0.150 and 0.155 at 30
and 50 °C, respectively) for both the rehydration temperatures.
3.4. Modeling of rehydration kinetics
Table 3 shows the values of various constants of Peleg’s model, first-order kinetic model (FOKM), and
Weibull model along with some other statistical parameters. For Peleg’s model, the values of both k1 (20.638
to 10.728 for DSD samples and 15.696 to 13.369 for HAD samples) and k2 (1.218 to 1.193 for DSD samples
and 1.221 to 1.215 for HAD samples) were found to decrease with an increase in rehydration temperature.
This trend was found to be the same for both DSD and HAD samples. As the values of k1 decreased with the
temperature increment (from 20.638 at 30 °C to 10.728 at 50 °C for DSD samples and from 15.696 at 30 °C
to 13.369 at 50 °C for HAD samples) at initial moisture content, therefore, the water transfer increased with
an increase in temperature. Analogous results were explained for different products by researchers with regard
to the rehydration[23,31–33]. Values of k2 were also found to decrease with temperature (from 1.218 at 30 °C to
1.193 at 50 °C for DSD samples and from 1.221 at 30 °C to 1.215 at 50 °C for HAD samples), indicating that
water absorption capacity increase with temperature. In the case of the first-order kinetic model, the parameter
kr is found to be decreasing with an increment in rehydration temperature for both DSD (0.043 to 0.032) and
HAD (0.040 to 0.039) apple slices. This decrease in the model parameter indicates an increment in the capacity
to absorb water with an increase in rehydration temperature. This result is in accordance with other researches
during the rehydration modeling of cassava chips[34]. Weibull model parameters also showed a similar trend
and both the parameters, i.e., α (0.983 to 0.928 for DSD samples and 1.013 to 0.904 for HAD samples) and β
(32.379 to 21.758 for DSD samples and 27.015 to 23.939 for HAD samples) were found to decrease with an
increase in rehydration temperature. The drop in the parameters clearly explains the increment in the capacity
of apple slices to absorb water with an increase in rehydration temperatures.
Table 3. Statistical parameters of modeling of rehydration kinetics of apple slices.
Model Rehydration Temperature (°C) Product
k1 k2 kr α β R2 RMSE
χ2
Peleg 30 SD 20.638
1.193
- - - 0.9833
0.0088
0.0319
HAD 15.696
1.221
- - - 0.9762
0.0104
0.0341
50 SD 10.728
1.218
- - - 0.9793
0.0095
0.0269
HAD 13.369
1.215
- - - 0.9823
0.0096
0.0249
FOKM 30 SD - - 0.032
- - 0.9989
0.0023
0.0015
HAD - - 0.039
- - 0.9962
0.0042
0.0048
50 SD - - 0.043
- - 0.9959
0.0042
0.0062
HAD - - 0.039
- - 0.9970
0.0039
0.0049
Weibull
30 SD - - - 0.983
32.379
0.9994
0.0015
0.0008
HAD - - - 1.013
27.015
0.9977
0.0029
0.0031
50 SD - - - 0.928
21.758
0.9985
0.0023
0.0015
HAD - - - 0.904
23.939
0.9993
0.0017
0.0007
The variation in experimental v/s predicted moisture content of Peleg’s model, FOKM model, and
Weibull model as attained from the solar dried and hot air-dried slices rehydrated at 30 °C and 50 °C have
been depicted in Figure 7 and Figure 8, respectively. For the Peleg model, the value of R2 was above 0.9762,
and RMSE and χ2 values were below 0.0104 and 0.0249, respectively for the rehydration of DSD and HAD
11
samples. For the FOKM model, the value of R2 was above 0.9959, and RMSE and χ2 values were below 0.0042
and 0.0062, respectively. The results displayed that the Weibull model has the maximum R2 values above
0.9977 and minimum RMSE and χ2 values below 0.0029 and 0.0031, of the three mathematical models which
have been applied to the rehydration kinetics of Fuji apple slices. The rehydration kinetics of Fuji apple slices
dried in both sun and hot air dryers are best described by the Weibull model, which was found to be the best
fit.
Figure 7. Variation in experimental v/s predicted moisture content of Peleg’s model (a) and (b), First order kinetic model (c) and (d)
and Weibull model (e) and (f), respectively as obtained from solar dried and hot air dried sample rehydrated at 30 °C.
12
Figure 8. Variation in experimental v/s predicted moisture content of Peleg’s model (a) and (b), First order kinetic model (c) and (d)
and Weibull model (e) and (f), respectively as obtained from solar dried and hot air dried sample rehydrated at 50 °C.
3.5. Color and texture measurements
The various color indices of rehydrated apple slices that have been measured are listed in Table 4. The
rehydrated HAD samples were found to retain more colors than those of DSD samples in terms of lower ∆E
and higher C* and h*. The reason behind this can be the direct exposure of the sample during drying in DSD
while HAD samples were dried using hot air only at indoor conditions. For samples that were rehydrated at
30°C, the ∆E, C* and h* values were 13.288, 15.870, and 89.517 for DSD samples and 11.884, 32.990 and
95.577 for HAD samples, respectively. Similarly, the values of ∆E, C* and h* for samples rehydrated at 50 °C
were 19.636, 13.707, and 66.720 for DSD and 16.141, 18.750 and 88.057 for HAD apple slices, respectively.
13
All these results indicate that HAD samples upon rehydration retained more color than DSD samples which is
in line with the results found by previous researchers[7].
a,b,c,d: Values in the same row with different letters represents significant difference (p ≤ 0.05).
The hardness and chewiness of rehydrated apple slices dried in DSD and HAD have been represented in
Table 5. It was seen that the required force to break the samples rehydrated at 30 °C was 15.087 N and 13.467
N for DSD and HAD samples, respectively. Similarly, the force required to break the samples rehydrated at
50 °C was 14.547 N and 13.323 N for DSD and HAD samples, respectively. Therefore, it can be deduced that
for both the rehydration temperatures, the hardness of DSD apple slices was seen to be more than HAD apple
slices. This trend might be because upon rehydration DSD apple slices had more dry matter holding capacity
values than HAD apple slices. The presence of more dry matter led to more hardness in DSD apple slices. The
chewiness value for the DSD apple slices was also seen to be higher than that of HAD slices which indicates
a better mouth feel. The DSD and HAD rehydrated samples have chewiness values of 7.033 mJ and 4.333 mJ
at 30 °C and 5.433 mJ and 4.133 mJ at 50 °C, respectively. This indicates that the DSD apple slices were better
in terms of texture as compared to the HAD apple slices. The decrease in the values of both hardness and
chewiness with increasing rehydration temperature is due to more leaching with an increase in rehydration
temperature, as a result, more soluble solids tend to move out from the sample matrix.
Table 5. Textural properties of rehydrated Fuji apple slices.
Rehydration temperature (°C) Samples Hardness (N) Chewiness (mJ)
30 SD 15.087 ± 2.29a 7.033 ± 1.01a
HAD 13.467 ± 1.09ab 4.333 ± 3.69ab
50 SD 14.547 ± 3.79abc 5.433 ± 2.25abcd
HAD 13.323 ± 0.84a 4.133 ± 2.71abc
a,b,c,d: Values in the same column with different letters represents significant difference (p ≤ 0.05).
4. Conclusions
The current research showed a significant effect on the drying time, rehydration ability, color retention,
and texture of Fuji apple slices due to the selection of pre-treatments before drying along with different drying
methods. The HAD samples have dried faster than DSD samples because of faster moisture removal due to
forced convection. The DSD samples on the other hand suffered less structural damage while drying and hence
they attained more equilibrium moisture content at the end of the process of rehydration as compared to HAD
samples. Additionally, DSD samples had better dry matter holding capacity values (0.300 and 0.286 at 30 and
50 °C, respectively) indicating more retention of soluble solids (nutrients). Rehydrated DSD samples having
been dried under direct sunlight were found to lose color properties significantly as compared to rehydrated
HAD samples in terms of overall color change ∆E. Because of having higher dry matter holding capacity
values, DSD samples were found to have higher hardness (15.087 N, 14.547 N at 30 and 50 °C, respectively)
and chewiness (7.033 mJ, 5.433 mJ at 30 and 50 °C, respectively) values which are an indication of better
Table 4. Color indices of rehydrated Fuji apple slices.
Color
indices
Rehydration temperature (°C)
30 50
SD HAD SD HAD
L* 67.800 ± 1.19b 72.123 ± 1.98a 62.100 ± 2.07cd 62.263 ± 1.28c
a* 0.020 ± 0.96b
–
3.173 ± 1.01a 5.343 ± 1.04d 0.573 ± 1.72bc
b* 15.853 ± 3.05bc 32.820 ± 2.15a 12.563 ± 1.33cd 18.687 ± 1.16b
∆E 13.288 ± 2.68ab 11.884 ± 0.94a 19.636 ± 2.64cd 16.141 ± 1.72bc
c* 15.870 ± 3.06bc 32.990 ± 2.09a 13.707 ± 0.83c 18.750 ± 1.15b
h* 89.517 ± 3.23b 95.577 ± 1.93a 66.720 ± 6.14c 88.057 ± 5.18b
14
mouth feel. Considering all the above results, it can be concluded that DSD samples were found to have better
rehydration characteristics and quality parameters (except color) as compared to HAD samples.
Author contributions
Conceptualization, DD and KD; methodology, DD and KD; validation, DD and KD; formal analysis, DD
and KD; investigation, DD and KD; resources, DD; data curation, DD and KD; writing—original draft
preparation, DD and KD; writing—review and editing, KD; supervision, PPT; project administration, PPT;
funding acquisition, PPT. All authors have read and agreed to the published version of the manuscript.
Conflict of interest
The authors declare no competing interests.
Abbreviations
ANOVA: Analysis of variance
d.m.: Dry matter
db: Dry basis
DHC: Dry matter holding capacity
EMC: Equilibrium moisture content
FOKM: First order kinetic model
HAD: Hot air dried
k1: Coefficient in Peleg model (min kg d.m./kg water)
k2: Coefficient in Peleg model (kg d.m./kg water)
kr: Rehydration rate of first order kinetic model (FOKM) (/min)
MR: Moisture ratio
RA: Rehydration ability
RR: Rehydration ratio
DSD: Solar dried
WAC: Water absorption capacity
wb : Wet basis
α: Shape parameter of Weibull model
β: Scale parameter of Weibull model (min)
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